Common Public Radio Interface CPRI transport over optical networks is gaining interest among operators that see it as an opportunity for real estate consolidation and leaned fibre cost saving and, in a longer term perspective, for the realization of an unified transport network spanning from radio front-haul to aggregation and backhaul of fixed and mobile access networks.
The transport technology for the optical transport network, OTN, has been standardized in ITU-T Recommendation G.709 and offers several advantages with respect to the previous generation of SONET/SDH networks, including stronger forward error correction, FEC, multiple levels of tandem connection monitoring, TCM, to communicate and manage error events across multiple domains, transparent transport of different client signals, including Ethernet, SDH, Fibre Channel, FICON, etc., and enhanced switching capability. At present, OTN is by far the most used technique to map any kind of client signal into an optical channel so that at a first glance it seems the natural choice also for CPRI.
OTN mapping of CPRI clients has been proposed and is included at Appendix VIII of G.709/Y.1331 (02.2012) but the standardization work is not yet complete due to three critical technical issues arising from demanding CPRI specifications, namely: jitter, absolute propagation delay and link symmetry. The CPRI jitter specification is 2 ppb r.m.s. phase noise, while current OTN practice is 300 Hz (equivalent to approximately 6 ppb r.m.s. phase noise). The mismatch in phase noise specification could be solved either by modifying the radio equipment to accept higher input noise, significantly increasing its cost, or by modifying the OTN standard to work at a lower phase noise specification, which could be achieved by introducing highly accurate oscillators and filters. Both of these proposed solutions options are business critical, due to the presence of an existing large installed network. Depending on the implementation, CPRI is expected to tolerate about 100 μs absolute propagation delay, which corresponds to 20 km fibre distance. OTN tributaries, e.g. Ethernet, usually do not have strict latency requirements, so OTN nodes and networks do not normally meet the tight latency specification that CPRI requires. Finally, downstream/upstream, DS/US, delay asymmetries introduced by optical link and buffering mechanisms in OTN processors are scarcely tolerated by CPRI, which requires a dedicated compensation mechanism or special optical design, such as bidirectional propagation on a single fibre, to cope.